Humphry Davy

“ The ground to which the colours are applied in the ancient fresco paintings, is precisely such as is described by Vitruvius, powdered marble cemented by lime, highly polished and beautifully white. With regard to the mode in which their colours were applied, Vitruvius and Pliny agree as to the employment of wax in encaustic painting, which was subsequently liquefied by heat so as to give a varnish to the painting. ”

“ If a piece of wire-gauze be held in the flame of a lamp, or of coal gas, no flame passes through the gauze; for though a portion of the inflammable vapour passes, it loses too much heat in its passage to propagate the flame; but in the case of inflammable mixtures of coal gas entering a lamp, and burning at the interior surface, that which is exterior has not been exposed to any heating cause, and consequently is in no danger of taking fire at the sides of the lamp ”

“ With a view to explain the non-transmission of inflammation through small apertures, the author considers the nature of flame in general; and since a piece of phosphorus, or even a small taper, will burn in the midst of a large flame made by the combustion of alcohol, he is of opinion that oxygen exists in the centre of all flame, forming an explosive mixture with the vapour, but which burns solely at the exterior surface, because it is there alone sufficiently heated to take fire. ”

“ Even the most explosive mixture will not take fire from well burned charcoal, nor by iron heated to any degree short of actual combustion. But on the contrary, hydrogen whether simple or sulphuretted, olefiant gas, or carbonic oxide, may each be set on fire very readily by all these means ”

“ A small quantity, not exceeding fifty or sixty grains, of the hyperoxymuriate are to be mixed with a small quantity of the acid in its concentrated state, and to be rubbed together by means of a spatula of platina till incorporated into a solid mass of a bright orange colour. ”

“ From the description which Pliny gives of an inferior sort of vermilion, formed by calcining certain stones found in veins of lead, the author is of opinion, that the mineral thus treated must have been a natural carbonate of lead, which becomes red when burned. ”

“ This oxide also combines with acids, forming compounds which crystallize. These are intensely acid to the taste, they redden vegetable blues, and act strongly on all metals. The effects of heat upon them are various, according to the different nature of the acid with which they are combined. The nitrate and sulphate may be sublimed unaltered, but are liable to partial decomposition if too suddenly heated. The oxalate is immediately and entirely decomposed by a gentle heat, and yields iodine and carbonic acid. ”

“ When the gauze is made of wire one fiftieth of an inch in diameter, and at intervals of one tenth, so as to make 100 apertures in the square inch, explosion may take place, either from intense ignition of the top of the lamp, or from lateral currents of air forcing the flame through the interstices. ”

“ In either case its durability, even in the interior of the mass, is a very curious circumstance, although the part exposed to the air has suffered the changes to which such colours have been too often proved to be liable, and accordingly no traces of it remain in any of the ancient fresco paintings.
All the blacks observable in the baths of Titus or elsewhere accord with the descriptions given by ancient authors, who speak of them as carbonaceous substances, obtained either as common charcoal or as soots of woods or resins. ”

“ If a lighted lantern, in the bottom of which there is only a small aperture just sufficient for its supply, be inserted in a mixture of common air, with about 1/12th of fire-damp, the flame first appears feeble, next enlarges, and then is uniformly extinguished. ”

“ The solution acts upon all metals, even upon gold and platina, and decomposes many metallic solutions, occasioning insoluble precipitates from solutions of lead or mercury, which are oxides of those metals. ”